Spark ignition engines typically have a gas pedal that is connected to an air throttle that meters air into engine. Stepping on the gas pedal typically opens the air throttle, which allows more air into the engine. In some cases, a fuel injector controller adjusts the fuel that is provided to the engine to maintain a desired air/fuel ratio (AFR). The AFR is typically held close to a stoichiometric ratio to produce stoichiometric combustion, which helps minimizes engine emissions and allows three-way catalysts to simultaneously remove hydrocarbons, carbon monoxide, and oxides of nitrogen (NOX).
Compression ignition engines (e.g. diesel engines) typically do not operate at stoichiometric ratios, and thus greater emissions and different emission components often result. Because diesel engines are now making real headway into the car and light truck markets, federal regulations have been passed requiring more stringent emission levels for diesel engines.
Unlike spark ignition engines, the pedal of a diesel engine is typically not directly connected to an air throttle that meters air into engine. Instead, in diesel engines with electronic fuel injection (EFI), the pedal position is sensed by a pedal position sensor, and the sensed pedal position is used to control the fuel rate provided to the engine, which allows more or less fuel per fuel pump shot. In many modern diesel engines, the air to the engine is typically controlled by a turbocharger, often a Variable Nozzle Turbocharger (VNT) or waste-gate turbocharger.
In many diesel engines, there is a time delay, or “turbo-lag”, between when the operator moves the pedal—injecting more fuel—and when the turbocharger spins-up to provide the additional air required to produce the desired air-fuel ratio. This “turbo-lag” can reduce the responsiveness and performance of the engine, and can increase emissions from the engine.
There are typically no sensors in the exhaust stream of a diesel engine that are analogous to those emissions sensors found in spark ignition engines. One reason for this is that diesel engines typically operate at about twice as lean as spark so ignition engines. As such, the oxygen level in the exhaust of a diesel engine can be at a level where standard emission sensors do not provide useful information. At the same time, diesel engines typically burn too lean for conventional three-way catalysts. As such, control over combustion in a diesel engine is often performed in an “open-loop” manner, often relying on engine maps or the like to generate set points for the intake manifold parameters that are believed to be favorable for acceptable exhaust emissions.
In any event, after-treatment is often required to help clean up exhaust emissions in a diesel engine. In many cases, after-treatment includes a “flow through oxidation” catalyst system, which typically does not have any controls. Hydrocarbons, carbon monoxide and most significantly those hydrocarbons that are adsorbed on particulates can sometimes be cleaned up when the conditions are right. Some after-treatment systems include particulate filters. These particulate filters, however, must typically be periodically cleaned often by burning off the soot particulate which has been collected on the filter to “Regenerate” the filter surface. Increasing the exhaust gas temperature is the primary way to initiate Regeneration, and injecting additional fuel in-cylinder or into an exhaust burner is one method. The control of this type of after-treatment may be based on a pressure sensor or on distance traveled, often in an open loop manner.